I’ve got my eyes set on the Sigma 10-20 mm f/3.5 EX DC HSM.
Here the lens is about 10% cheaper than the Canon EF-S 10-22, so the price isn’t that different. I’ve searched the forum for references for the sigma lens but I’ve found only the exhaustive review that Thomas wrote about the sigma 10-22 f/4-5.6. The Canon has 2mm more reach but the sigma is faster. I’d like that lens for indoors shots as well as landscape, architecture and street photography. I’m not sure which one to buy.

Thank you Razvan, now I have another option. The Tokina is on the same price as the Canon 10-22. I'ts very tempting acually, the only fly in the ointment is that it doesn't reach 20mm that would give me 30ish mm for the classic landscape lens. One alwais can crop though. Another thing is that geometric distortion is larger than the Canon and Sigma. Nothing is perfect Tough to choose.

Some lenses have odd distortion profiles which can be tricky to correct. I'm not saying the Tokina should be written off, but I think Radu is right to take that into consideration especially with architecture.

Radu, I see you already have a 17-85 so I wouldn't think that extra 2mm or even 6 between the Tokina/Sigma and Canon is that important unless you plan on leaving the house with only one lens. I almost always carry my 10-20 with my 18-135 in the bag so for me, any overlap wouldn't really be missed.

The old Sigma 10-20 (variable aperture) I did like while I had it. It wasn't critically sharp wide open but sharpens up nicely stopped down. It was highly usable without distortion correction, although technically the edges exhibit a less regular effect which is hard to correct if you're looking at that level of detail. I didn't do architectural work, but you don't notice it in natural landscape.

On the Tokina, nominally 11mm is 10% less wide than 10mm. If you're going for an ultra-wide, that is worth considering.

While I'm generally happy with my Tokina 11-16, I find it prone to rather unflattering flaring, even with the petal-shaped hood attached.

Granted, I like to shoot with the sun in the picture - but even with the sun out of the picture, it's fairly annoying. Compared to something like the 10.5mm Nikon Fisheye (also F2., it's a dog when it comes to flaring.Never used the Sigma though, so it may be just as bad and of course, a fish-eye is not a rectilinear wide-angle at all.

If you can avoid the flares, the Tokina is a great "party lens", making rooms and indoor locations look roomy and expansive.

Thanks guys for the answers.
@grahamnp
I plan to “complete” the lens system by selling the 17-85 and buying an ultra wide, an EF 24-105 L and an EF 70-200 L. Then with my 2 primes, 50 f/1.4 and 100 f/2.8, and the lenses above I may rest for a while. Because I’d like to have the 24-105, and in the absence of the 17-85, I think I’ll need something that gives me the equivalent 30-35mm.
@popo
Thanks for pointing that out (the 10% difference). In the projects that I have in mind that could be significant.
@LahLahSr
I really appreciate the caveat about the flares, here there is sun everywhere 8 months a year, sometimes it feels like the sun is a double, and one can’t get away from it.
So in conclusion about Tokina
+ sharp
+ f/2.8
-10%less
-flares
-more geometry distortion
-less reach
Hmmm, it looks like only Sigma and Canon remain in competition.
The main thing that makes me hesitate concerning the Sigma is what I’ve read in Thomas’ review about QC.
With the dealers here it’s hard to test a lens before you buy and returning it is problematic at best when it’s about IQ and not some physical/mechanical fault.

This was my first time UWA Lens , i loved it , it has solid built quality and quite light compare to my Canon EF-S 17-55mm .

This lens has 82mm filter size , yeah it's a big and scary , so i must grep my pocket again another SGD170 to buy filter . For the one , i choose B+W 82mm XS-Pro Clear MRC , it must apply with thin / slim sized filter , in order to protect from flare and fit with the lens cap .

About the image quality , so far i'm impressed . Especially , i can take the shot on indoor area .

odd distorsions or not,I think they can be corrected. for a small price difference,the extra sharpness & F/2.8 are a bargain. all wide angles have pretty weird distorsions. (the king & queen of this category would be the sigma 8-16mm)

Actually, they can't be fully corrected with traditional distortion correction tools. My own Sigma's "moustache" distortion is completely left unchanged by distortion correction, as I would expect. It's not a big deal, I mostly use it for landscape where it usually goes unnoticed but it can be real pain for somebody more discerning than me or possibly architecture.